General opinion on 1275 compression ratio

K

kipsy

Hola, I've been told that my micra needs to have the compression ratio raised to increase the torque of my engine. The car has a flat spot/torque dip at 4000 rpm. The car has 1.6 inj, cup cams (?) fse regulator, exhaust n gpa filter and is making 95@ wheels. I was told by my rolling road operator to skim 23 thou off the head as per what bs does on the 1000 cc micras. When having a conversation on here with a very helpful member he said that 23 thou is only a small amount and it won't make much difference. Just wondering how much have people been skimming their heads and what compression ratio they are getting? My bottom end is standard I believe. Cheers
 
Frank will have a lot more detail for you but I believe you can buld up a high comp 1275 using a CG10 block and pistons, with a CG13 crank and rods.
 
Frank was the really helpful member (Y) but just thought I'd ask the other members. I ideally don't want to rebuild the entire engine but just pop the head off and give it a skim. Got rally on epynt the end of this month and going to do it after that. Putting an entry in for Wales rally gb at national level so need all the help I can get!
 
Edit: If you don't want to strip the engine down then it's either head off and skim or try a thinner gasket, (which may very well be more expensive than a head skim). So I think that pretty much answers the question......skim it and see I guess.

As a follow up, we're putting a CG13DE together with some CG10 pistons plus a few other refinements shortly, however I expect you may get there before we do, (not looking to complete the build and map it up till mid September). Power figures should be interesting however but perhaps slightly more scope for improvement seeing as we are running ITBs. I am sure the results will be available through the Fusion Motorsport section on the forums when done, if you are interested in the gains.
 
My compression has been down of late, due to ongoing ill health, but that's another story :eek:

You're not going to get better/more advice than you've already received from Frank in the other thread.

This is well worth a read too http://www.micra.org.uk/threads/40439-quot-Maxi-Micra-quot-project....by-EBBdude.

oh and don't be shy with a thank you every now and then, nobody likes putting a bit of work in with little or no recognition, and yes I'm grumpy in the mornings .........

Edit: you two got two posts in whilst I searched for that thread, nice to see Kipsy sticking his thumb up (Y) although I hate that little man myself, the one with the thumb that is :wasntme:
 
Yes lowrider I would be interested in power figures once done. Bit of an addict with rolling roads. If my car could get 125 bhp with power coming in from around 3 onwards that would be nice without the torque dip. As the car has a 5.1 cwp fitted it should accelerate pretty good then ( for a 1.3(Y) )
Cheers for the advice peeps
 
Spot on thankyou Frank! If I was to put 1 l flat top pistons in with 0 dome the comp ratio would rise to 11.74-1. That would be a nice increase don't you think. Ill becontacting the original owner of my micra later to pick his brains on what's actually been done to the engine. It may be a case of just putting 1 litre pistons in from the bottom and leavin the head on? Don't know if that's possible on these. Not far from the 12-1 target
 
kipsy said:
Spot on thankyou Frank! If I was to put 1 l flat top pistons in with 0 dome the comp ratio would rise to 11.74-1. That would be a nice increase don't you think. Ill becontacting the original owner of my micra later to pick his brains on what's actually been done to the engine. It may be a case of just putting 1 litre pistons in from the bottom and leavin the head on? Don't know if that's possible on these. Not far from the 12-1 target
Click to expand...

no mate, its a head off job, and the wristpins are an interference fit :eek: http://www.micra.com.au/community/message.php?messageid=65695
and a 1mm headskim = about 4cc less in the combustion chamber btw
 
Quick calculation, as you stated Frank 23 thou isn't much and that would only up the comp ratio to 10.1-1 . Where as a 2 mm skim takes it to 11.71-1. Much better. Or now with 1.0 l pistons 15-1 boom.
Thanks again. Interesting
 
Right, re calculation then. 1mm head skim, - 4 cc in combustion chambers (21) = 11.72 -1. Spot on ideal ratio. 1mm skim sounds ideal
 
I typed in 1mm skim so just put the gasket as .2 instead of 1.2 and minus 4cc per cylinder so 21 cc and calculator showed 11.42
 
kipsy said:
I typed in 1mm skim so just put the gasket as .2 instead of 1.2 and minus 4cc per cylinder so 21 cc and calculator showed 11.42
Click to expand...

nah, a 1mm block skim would be 4cc (1 cyl = 319cc, divided by the 80.5 stroke = 4cc per milimetre of stroke )
but a headskim is a bit less actually (not a full circle, with the quench steps)
mine works out to 12:1 eh, with the 82.8mm CGA stroke :)
 
you just take 3.5 to 4cc off the combustion chamber figure eh, and if yours has the nme challenge cams kipsy, i believe they are 254 deg and 9mm lift btw
 
So was my calculation wrong sorry? I understand what your saying about the head not being round that makes total sense when thinking about it. Are the cams I have fitted any good then? Do you think they are capable of producing more bhp and torque with the compression increased. I know a few pintos need to run high compression ratios with certain cams but unsure about the micra lump. When I was speaking to my rolling road operator I'm not sure he realised the the 1275 motor has domed pistons. I used the calculator to work out the approx gain a 1L engine would gain from a 23 thou skim and it ups it to about 11.3 which is a nice increase but wouldn't work as you explained on the 1275. Thankyou for all your help it's really getting me into this engine.(Y)
 
i,ve no experience of matts cams at all mate, (only reprofiled ones), you,re best to ask lowrider tbh. (i think he has used both types)
matt has only got the 264 rally ones listed on his site now i see
 
very interesting so this would be a 1mm head skim and the cg10 pistons giving around 12:1 CR which is a happy CR anymore and cold lead to problems!
 
NME challenge cams are 254deg duration, 8.35mm lift and timings of 17/57/57/17. The 773 Rally cams are 264deg duration, 9.09mm lift and timings of 26/58/58/26. Both of which will be too tame for our next build but we'll see what they make anyway. I have a set of blanks sitting in a box, so they maybe bought out for some special grinds at some point or shelved and a set of billets made instead.....we'll see what occurs ;)
 
Just to confirm guys, 1.0l pistons are exactly the same size as 1275 pistons minus the dome on the compression face. The pins fit and 1275 rods fit together. This seems the easiest option to up the compression ratio with out playing with the timing chain etc.
 
kipsy said:
Just to confirm guys, 1.0l pistons are exactly the same size as 1275 pistons minus the dome on the compression face. The pins fit and 1275 rods fit together. This seems the easiest option to up the compression ratio with out playing with the timing chain etc.
Click to expand...

yes mate, the 1.0 and 1.3 are round skirted and have thicker rings and the coilpack ones are oval skirted with thinner rings and ringlands, but the wristpins are a mission to remove/refit tho :eek:
 

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